Dischargers for powders

ABSTRACT

There is disclosed a discharger ( 100 ) for a powder, the discharger comprising a hopper ( 102 ) and an outlet ( 8 ), the hopper comprising a discharging surface ( 5 ) over which, in use, a powder flows, and means ( 14, 15 ) for lowering the angle of internal friction of the powder or for lowering the angle of sliding friction between the powder and the discharger surface, wherein the discharging surface comprises surfaces at least two angles that are progressively shallower towards the outlet and in which a steeper part of the discharging surface in use is steeper than 40° from the horizontal. A method of use of such a discharger is also disclosed.

FIELD OF THE INVENTION

The present invention relates to dischargers for powders.

In particular, but without limitation, the present invention is relevantto the field of storage bins for powders, such as wheat flour.

BACKGROUND TO THE INVENTION

Dischargers for powders generally comprise a hopper and an outlet.

Several aspects must be considered in the design of hoppers. First, theproduct in the hopper must not form an arch, which interrupts the flowof the product. If and when the arch collapses the product will oftensurge from the hopper. It is well known that arching can be eliminatedif the opening at the outlet of the hopper is large enough. For a rightcircular conical hopper, the critical gravity flow-arching dimension fora particular material is designated as Bc. As will be seen below, thepresent invention permits the use of a discharge opening that is only afraction of Bc.

A second consideration in the design of hoppers is that the hopper wallsmust be steep and smooth enough to force the product to slide on themwhenever any is withdrawn from the outlet. If the hopper walls are notsteep enough the product will form a very steep sided flow channelwithin otherwise stagnant material, which results in many undesirableconsequences. For a hopper having the shape of a right circular cone,the largest semi-apex angle at which mass flow will occur for aparticular product and hopper wall condition is denoted by θ_(c). Ingeneral, there is no advantage to making the walls of a hopper steeperthan the angle required for mass flow. In fact the theory teaches thatthere is an optimum slope angle (for mass flow). Making the slopesteeper, as well as less steep, decreases the flowability of the hopperaccording to this theory.

“Mass flow” is defined and the method for the determination of a slopeangle required for mass flow is set out in “Storage and Flow of Solids”by Andrew W Jenike; Bulletin 123 of Utah Engineering Experiment Station.This also provides the method for determining the critical archingdimensions Bc and Bp.

A design that is used occasionally when the product being handled is afine, dry powder with low permeability consists of a right circularconical hopper with a plenum chamber covered by a permeable membrane.Air blown into the plenum chamber passes through the permeable membraneand fluidises the powder, removing its ability to form an arch. Theslope of the right circular conical hopper is not normally as steep asθ_(c) since the fluidised powder flows like a fluid rather than apulverulent solid.

The advantages of ‘plane flow’ as opposed to ‘conical flow’, are wellknown. Plane flow occurs when a particulate solid flows through a‘slotted’ hopper outlet with the length of the slot being at least threetimes greater than the width. The theory teaches that the criticalgravity flow arching dimension for a slot outlet (Bp) is approximatelyhalf that for a right circular conical hopper outlet (Bc). In addition,the critical semi-apex hopper angle at the side of a slot (θ_(p)) isapproximately 10° to 12° larger than the semi-apex angle for a rightcircular cone (θ_(c)). In spite of the advantages of plane flow, it isnot often used in practice because the problem of collecting the productbelow the slot outlet can be relatively complex.

A further consideration in the design of hoppers is the optimisation ofthe geometry of the hopper within the constraints described above.Normally, in most applications it is preferred to use, for a givenvolume, a hopper that is shortest in height.

A design that has been used to create a plane flow pattern makes use ofa relatively narrow strip of permeable membrane, rather than a rightcircular truncated cone, above a plenum chamber. The strip, whose widthis set by Bp, the gravity flow-arching dimension of the powder for planeflow, has a length greater than three times the width of the strip. Theslope of the membrane is normally less than θ_(p). At its lowest pointon the centreline, the strip has an outlet, which also contains airjets. In such designs it is necessary to divide the plenum chamber intoairtight sections so that different amounts of air can be blown intodifferent regions of the bin.

One of the reasons collecting material under a slot is complex is thatthe product must shear in a certain way and change its direction offlow. Development of the correct shear planes and the forces required tochange the direction of flow can be difficult to achieve.

SUMMARY OF THE INVENTION

According to the present invention in a first aspect, there is provideda discharger for a powder, the discharger comprising a hopper and anoutlet, the hopper comprising a discharging surface over which, in use,a powder flows, and means for lowering the angle of internal friction ofthe powder or for lowering the angle of sliding friction between thepowder and the discharger surface, wherein the discharging surfacecomprises surfaces at at least two angles that are progressivelyshallower towards the outlet and in which a steeper part of thedischarging surface in use is steeper than 400 from the horizontal.

By reducing the angle of internal friction or the angle of slidingfriction and providing a sloped wall in the range of angles specified,surprisingly it has been found that many of the problems referred toabove are done away with and a relatively small outlet can be used. Inthis way the mass flow angle is less dependent on powder properties anda reliable flow regime can be produced from such an arrangement.

Suitably, the discharger surface is steeper than 45°, and more suitablysteeper than 500 from the horizontal.

Suitably, the discharger surface slope is steeper than 600 from thehorizontal.

Suitably, the width of the discharger surface is less than 30% of itslength.

Suitably, the discharger surface is curved from the back to the outlet.

Suitably, there is provided means for lowering the angle of internalfriction and for lowering the angle of sliding friction between thepowder and the discharger surface, in which means the discharger surfacecomprises a gas permeable membrane and means for passing a gas throughthe gas permeable membrane.

Suitably, the gas permeable member comprises a flexible bag.

Suitably, means is provided to pass a gas through a powder from beneath.Suitably, the discharger is adapted to pass gas through a powderadjacent to the outlet.

Suitably, there is provided means for lowering the angle of internalfriction and for lowering the angle of sliding friction between thepowder and the discharger surface, which comprises a vibrator forvibrating the discharger surface.

Suitably, the discharger further comprises a front wall adjacent theoutlet, which front wall slopes inwardly towards the hopper as itextends to the outlet.

Suitably, the inward slope is substantially 100 from the vertical.

Suitably, the discharger surface is curved across the dischargersurface.

Suitably, the curve is generally a U-shape.

Suitably, the discharger comprises side walls at least one of whichdiverges as it approaches the outlet.

Suitably, at least one internal ridge is provided extending from a sidewall.

Suitably, at least one ridge extends in a direction toward the outlet.

Suitably, the outlet is at the side of the discharger. In a generallyrectangular shaped discharger a side will be on a long edge.

According to the present invention of a second aspect, there is provideda method of use of a discharger according to the first aspect of theinvention, the method comprising discharging a powder through theoutlet.

It has been found that in dischargers according to preferred embodimentsof the present invention increasing the slope of a hopper in one regionbeyond 400 takes it beyond the critical angle required for mass flow,but improves the flowability of the hopper. Further, a steep portion ofthe hopper, combined with an optimised hopper layout, occupies lessheadroom than would be required for a conventional mass flow hopperdesign, especially when the angle of sliding friction is high. Inpreferred embodiments, this hopper layout improves the flowability ofthe hopper to the extent that the outlet size need only be a smallfraction of Bc, the critical gravity flow arching dimension of a rightcircular conical hopper. The flow through small outlet sizes is asignificant advantage when the powder has cohesive strength and alsowhen the control of flow rate is important such as in dispensingpharmaceutical powders or measuring out food products and ingredients.

With preferred embodiments of the present invention, one hopper designwill reliably handle powders with a very wide range of flow properties.The choice of hopper width and outlet size will be based more on therequired flow rate and accuracy of control than on the critical archingdimensions.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example only, withreference to the drawings that follow; in which:

FIG. 1 is a plan view of a discharger according to a first embodiment ofthe present invention.

FIG. 2 is a perspective view of the discharger in FIG. 1.

FIG. 3 is a front view of the discharger of FIGS. 1 and 2.

FIG. 4 is a side view of the discharger of FIGS. 1-3.

FIG. 5 is a part-sectional expanded view on the line A-A in FIG. 4.

FIG. 6 is a plan view of a permeable membrane bag for use with thepresent invention.

FIG. 7 is a perspective view of the bag of FIG. 6.

FIG. 8 is a side view of the bag of FIGS. 6 and 7.

FIG. 9 is a plan view of a discharger according to a second embodimentof the present invention.

FIG. 10 is a perspective view of the discharger of FIG. 9.

FIG. 11 is a front view of the discharger of FIGS. 9 and 10.

FIG. 12 is a side view of the discharger of FIGS. 9-11.

FIG. 13 is an enlarged, partly sectional view on B-B in FIG. 12.

FIG. 14 is a sectional side elevation of a discharger according to athird embodiment of the present invention.

FIG. 15 is an enlarged, sectional view on the line C-C in FIG. 14.

FIG. 16 is a sectional view on the line D-D in FIG. 15.

FIG. 17 is an enlarged partly sectional view of a discharge outlet.

FIG. 18 is a perspective view of an embodiment of the present inventionin use with a flexible intermediate bulk container.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIGS. 1-4 of the drawings that follow, a first embodiment of thepresent invention comprises a discharger 100 having a hopper 102 and anoutlet 8. The hopper 102 comprises an upper section 1 and a lowersection 2. Upper section 1 can be added to lower section 2 if required.The upper section 1 will ordinarily be required when the discharger 100is used as part of a bin, but may not be required in the form shown ifthe lower section is used as part of a flexible walled storage containeror other type of process equipment. Lower section 2 of hopper 102 is ofa rectangular box shape with rounded ends and comprises side walls 3, anend wall 4 and a discharger surface 5 as a lower wall defining agraduated curved surface with, in the normal operating orientationshown, a steep section 6 and a shallow section 7 at the lower endthereof, the latter extending to a discharger outlet B. Upper section 1of hopper 102 includes end walls 9 that diverge downwardly.

The sections 6 and 7 together comprise the discharger surface 5 on whicha powder in the hopper may slide. The discharger surface 5 is thenon-vertical section before end wall 9. The discharger surface 5 has acurved shape from steep section 6, progressively becoming a shallowerone to shallow section 7. The distinction between the two sections 6, 7is somewhat arbitrary, but useful for explaining operation of thepresent invention.

The sidewalls 3 of the lower section 2 are shown as vertical andparallel from back to front in FIGS. 1-4 and 9-12 but they may divergeupwardly. The front wall 4 may be vertical or near vertical. In thepreferred embodiment the front wall is sloping inward (toward the centreof the discharger) approximately 10° from vertical. This angle is shownas θ₃ in FIG. 4. In this configuration, the front wall forms atransition shape from the rounded shape at the top to a square shape atthe bottom of the front wall 4. The back and lower edges of the box arecombined to form the lower wall 5. The end of steep section 6 of thedischarger surface 5 is steeper than the angle θ_(p) and normallygreater than 40° from the horizontal. Ordinarily, the angle of the lowersection 7 of the discharger surface 5 is between 10° and 15° from thehorizontal. This angle is shown as θ₂ in FIG. 1. The curved shape ofdischarger surface 5 leads into the outlet 8 of the discharger, whichprojects beyond the front wall 4 by a horizontal distance approximatelyequal in magnitude to the vertical distance from the bottom of the frontwall to the ski-jump shaped surface. The angle of the end of lowersection 7 may be steeper than θ₂.

Referring specifically to FIGS. 5-8 of the drawings that follow theflexible bag 14 consists of a flexible air permeable membrane, such aswoven polyester, attached with an airtight attachment, such as glue, toa lower skin around its edges, incorporating a nozzle 15 so that air orgas may be blown into the space between the two skins. In someapplications the upper and lower skins of the bag are attached togetheralong lines 20 by means of glue or stitching, to minimise the deflectionof the membrane during operation. The discharger surface carries a gas(preferably air) permeable membrane in the form of a bag 14. (Thisarrangement is particularly useful when the membrane must be changed orwashed regularly as is the case when handling pharmaceutical and foodproducts where cross-contamination must be avoided.

With the curved shape of the discharger surface and the effect ofgravity pushing the powder along the full length of the dischargersurface it is not necessary to have sections in the plenum chamber. Oneair pressure giving an equal flow of air through every section of thepermeable membrane is sufficient to cause the powder to flow.

In the embodiment shown in FIGS. 1-5, the bag 14 rests on the curveddischarger surface 5 and is held in place between flanges 18 at thelower edge of the sidewalls 3 of the box and a plate 17 shaped to fitsnugly to the shape of flanges. The method of holding the plate to theflanges is spring clips 19 but any quick release clamp or other suitableattachment may be used. The spring clips are not shown in FIGS. 1-4, forclarity.

The flowability of the hopper is improved by having a downwarddivergence of the end walls 9 of the upper section as shown in FIGS. 1,4, 9 and 12.

This first embodiment is used when handling powders with lowpermeability, which can be conditioned by blowing air into the mass ofpowder. In the region of the outlet where the powder has an exposed,unconfined surface, a relatively low flow of air (or gas) substantiallyremoves the angle of internal friction from the powder in the outletregion, allowing it to flow. This also removes the powder's cohesivestrength—the property that gives it the ability to form an arch, so thatit flows through the outlet. In addition, the air or gas flow throughthe permeable membrane lowers the angle of sliding friction between thepowder and the discharger surface. With the resisting forces due tofriction substantially removed, gravity acting on the powder, especiallyin the steep section of the discharger surface, pushes the layer ofpowder in the region immediately above the shaped sliding surface towardthe outlet.

The amount of air required to move the powder is relatively low. Thiscombined with the small area of permeable membrane relative to thevolume of powder being moved leaves the powder in a ‘conditioned’ staterather than fluidised. This is an advantage since a fluidised powder canbe difficult to handle and its low bulk density makes it difficult topack.

Referring to FIGS. 9-13 of the drawings that follow, a second embodimentof the present invention is shown. Like reference numerals are used forcomponents similar to those in the FIGS. 1-8 embodiment. In this secondembodiment lower wall 12 is supported from the lower section 2 bysprings 11 (see FIG. 13) or other suitable resilient connectors so thatthe lower wall 12 may be vibrated. An electromagnetic vibrator (notshown), or other method common in the industry will normally apply thevibrations. The vibrator, its method of attachment to the lower wall andthe attachment springs are not shown in FIGS. 9-12, for clarity.

In the second embodiment of FIGS. 9-13, the lower wall 12 is shown ascurved, with a flattened U-shape, in its cross-section (see FIG. 11)i.e. across the discharger surface. Experience has shown that this tendsto direct the flow toward the centreline of the sliding surface andminimises the tendency of the powder to squeeze between the sidewall andthe vibrating surface during operation.

As shown in FIG. 13, an additional flexible strip 10 is shown mounted tothe sidewall to seal the necessary gap 13 between the sidewalls 3 andthe vibrating surface 12. The side walls 3 are shown as parallel to oneanother in plan in FIGS. 9-12 but they may diverge from the back to theoutlet to further relieve the confining pressures on the powder as itflows down the discharger surface 5.

This second embodiment will be used when handling powders that cannot beconditioned by blowing air into the mass if, for example thepermeability of the powder is too high. Both the removal of the angle ofrepose of the powder in the region of the outlet and the reduction ofthe angle of sliding friction between the powder and the dischargersurface are achieved by vibrating the discharger surface. Thesevibrations do not compact the powder, which is allowed to expand as itflows. In some applications it may not be necessary to vibrate the wholesurface if the powder will slide on the steep surface anyway.

Referring to FIGS. 14-16 of the drawings that follow a modification ofthe first and second embodiments is shown. In some applications it maybe difficult to create a uniform flow along the length of the curveddischarger surface 5 or 12. In this embodiment three equally spacedinternal ridges 21A, 21B, 21C are provided in the hopper 102. The ridges21 are located on the side walls 3 of lower section 2. The ridges 21project out from the side wall 3 a distance X, approximately 10% of thewidth W of the lower section. There may be more or fewer ridges indifferent applications. In a preferred embodiment, each ridge extendsfrom a line drawn, parallel to the discharger surface 5 at its lower end(θ₂ from horizontal), from the lower edge of the front wall 4 to a linedrawn horizontally from a point three quarters of the distance up thelower section 2, measured from the lower edge of the front wall 4. Theridges are fixed to the side walls on lines drawn radially from a pointvertically below the lower edge of the front wall a distance H, which isapproximately twice the distance from the lower edge of the front wall 4to the discharger surface 5. It has been found that these ridges improvethe uniformity of flow along the discharger surface 5.

FIG. 17 shows a valve 23 located in the discharger outlet 8 to controlthe flow rate of powder and seal the discharger when it is not in use orwhile it is being filled. The valve 23 is used in addition to othermethods of controlling the flow rate of powder from the hopper, such asvarying the amount of air being blown through the permeable membrane orvarying the frequency and amplitude of vibrations.

In FIG. 18 the lower section 2 of the discharger is shown attached to aflexible intermediate bulk container (FIBC) 22. The FIBC is shown astransparent in FIG. 18, for clarity.

Other angles of the steep section that can be advantageous are more than45°, 50° and 60° from the horizontal.

Dischargers of the type described herein are ordinarily fabricatedprimarily of sheet metal such as galvanised or stainless steel. However,the present invention is not limited to any particular material and mayin some circumstances be made of plastic and may even be flexible ratherthan rigid.

Thus, preferred embodiments of the present invention permit the use of adischarge opening that is only a fraction of Bc.

Thus, preferred embodiments of the present invention that incorporate amechanism to provide dischargers that temporarily alter the powder'sflow properties. The shape of a special sliding surface allows gravityto cause the powder to flow through an outlet significantly smaller thanwould be possible with other hopper shapes and with less verticalheadroom than existing designs especially when friction angles are high.

At the shallow end of the discharger surface, the powder properties aretemporarily altered so that its angle of internal friction or angle ofrepose is reduced. The powder flows readily through the outlet even ifit is substantially smaller than the theoretical minimum outlet size Bc(or Bp). In addition, the angle of sliding friction between the powderand the discharger surface is temporarily reduced to a low value.Gravity acting on the powder at the steep end of the discharger surfaceprovides the moving force that makes the powder flow. The higher thevertical forces acting on the powder at the top of the dischargersurface, the better the flow conditions in the hopper. Both of theseobjectives are achieved in the first embodiment by the air or gasflowing through the permeable membrane into the powder and in the secondembodiment by vibrating the discharger sliding surface.

In other hoppers such as a right circular conical mass flow hopper,higher forces than gravity acting on the powder at the top of thehopper, such as due to vibrations, will normally reduce the flowabilityof the hopper by wedging the powder into the converging shape.Vibration, impact or higher vertical load on the powder improves theflowability of powder in preferred embodiments of the present invention.This is an advantage since the natural response of operators when facedwith flow problems in a hopper is usually to impact or vibrate thehopper.

It is expected that one of the most useful applications of embodimentsof the present invention will be to discharge powders from FIBCs. Thelower section, which may be made of flexible material, is permanentlyattached to the underside of an FIBC. Before filling, storage andtransport, the lower section is folded and temporarily secured. When thepowder is required to be discharged, the lower section is unsecured fromits temporary folded position and allowed to fall into its loweredposition and fill with powder, which may then be discharged.

The description and drawings show embodiments of the invention with anoutlet section located to the side of the main body of the containervessel. This provides easy access to the control valve and dischargeinto some types of equipment but a centrally located outlet withdischarger surfaces on each side of the outlet are understood to be awithin the scope of the present invention. In addition, it is understoodthat a discharger surface made up of a number of straight sectionsjoined end-to-end and angled relative to one another is also within thescope of the present invention. In addition, the sidewalls of the lowersection may diverge upwardly to increase the volume of the discharger insome applications.

The reader's attention is directed to all papers and documents which arefiled concurrently with or previous to this specification in connectionwith this application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings), may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiment(s). The invention extend to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

1. A discharger for a powder, the discharger comprising a hopper and anoutlet, the hopper comprising a discharging surface over which, in use,a powder flows, and means for lowering the angle of internal friction ofthe powder or for lowering the angle of sliding friction between thepowder and the full discharger surface, wherein the discharging surfacecomprises surfaces at of at least two angles that are progressivelyshallower towards the outlet and in which a steeper part of thedischarging surface in use is steeper than 40° from the horizontal.
 2. Adischarger according to claim 1, in which the discharger surface issteeper than 45° from the horizontal.
 3. A discharger according to claim2, in which the discharger surface is steeper than 50° from thehorizontal.
 4. A discharger according to claim 3, in which thedischarger surface is steeper than 60° from the horizontal.
 5. Adischarger according to claim 1, in which the width of the dischargersurface is less than 30% of its length.
 6. A discharger according toclaim 1, in which the discharger surface is curved from the back to theoutlet.
 7. A discharger according to claim 1, In which there is providedmeans for lowering the angle of internal friction and for lowering theangle of sliding friction between the powder and the discharger surface,in which means the discharger surface comprises a gas permeable membraneand means for passing a gas through the gas permeable membrane.
 8. Adischarger according to claim 7, in which the gas permeable membercomprises a flexible bag.
 9. A discharger according to claim 7, in whichmeans is provided to pass a gas through a powder from beneath.
 10. Adischarger according to claim 9, in which the discharger is adopted topass gas through a powder adjacent to the outlet.
 11. A dischargeraccording to claim 1, in which there is provided means for lowering theangle of internal friction and for lowering the angle of slidingfriction between the powder and the discharger surface, which comprisesa vibrator for vibrating the discharger surface.
 12. A dischargeraccording to claim 1, in which the discharger further comprises a frontwall adjacent the outlet, which front wall slopes inwardly towards thehopper as it extends to the outlet.
 13. A discharger according to claim12, in which the inward slope is substantially 10° from the vertical.14. A discharger according to claim 1, in which the discharger surfaceis curved across the discharger surface.
 15. A discharge according toclaim 14, in which the curve is generally a U-shape.
 16. A dischargeraccording to claim 1, in which the discharger comprises side walls atleast one of which diverges as it approaches the outlet.
 17. Adischarger according to claim 1, in which at least one internal ridge isprovided extending from a side wall.
 18. A discharger according to claim17, in which the at least one ridge extends in a direction toward theoutlet.
 19. A discharger according to claim 1, in which the outlet is atthe side of the discharger.
 20. A method of use of a dischargeraccording to claim 1, the method comprising discharging a powder throughthe outlet.